Sanitary concerns as well as federal and state regulations in food processing and food preparation industries necessitate a device capable of rapidly and efficiently detecting various test samples from materials or surfaces. Various test apparatuses and test methods have been developed for that purpose. For example, it is widely desirable to determine or to test through quantitative and qualitative tests food, such as meat products, fruit, vegetables, and to detect for alkaline phosphates, salmonella, drugs, and antibiotics, such as; for example, various bacteria and pathogenic combinations, either in materials or on the surface of materials, or both.
The present commercial tests for the detection of ATP-luciferase reaction are generally directed to a chemiluminescence test, which ordinarily employs premeasured and prepackaged separate test reagents mixing with the test sample to produce chemiluminescence. Accordingly, the count corresponding to the concentration of the ATP, which is determined by measuring or counting of the chemiluminescence, is compared against certain accepted control standards, or a threshold of a control standard.
Photomultiplier consumable-based detectors have been typically used to monitor the ATP-luciferase reaction. A photomultiplier consumable or PMT, capable of responding on an individual photon-by-photon basis amplifies low-level light intensity generated by chemiluminescence. As any light amplifying device, PMT is a delicate device that is not particularly suited for use by an unsophisticated user typically employed in food processing and food preparation industries and is bulky and costly requiring employment of a complicated manufacturing process.
Another device capable of generating an output current in response to low-level light signals is a photodiode, which is known to generate currents substantially lower than PMT. To convert currents from PMT into a useful electrical representation, an analog front-end circuit, such as the transimpedance amplifier, has been employed in devices used to measure chemiluminescence. As shown in FIG. 1, a typical transimpedance amplifier uses resistance to provide a real time linear representation of the light source. Input current generated by a photo-detector flows through the feedback resistor, RF, to create a proportional output voltage VO=−IINRF. Accordingly, since RF determines the transimpedance gain (amplification), very large values (gigaohm) of the feedback resistance RF are required to measure small signal input current. However, transimpedance amplifiers with such high values of the feedback resistor are notorious for production problems—since the resistor and circuit board must be extremely clean to prevent stray feedback paths that otherwise will lower the gain of the amplifier, which can be detrimental for devices used to detect low level signals generated as a result of chemiluminescence. Also, to maintain the desirable cleanliness is even more difficult in the food production and food processing industries where such devices are employed.
Also, the dynamic range of the transimpedance amplifier is limited unless gain switching is employed. Such gain switching includes a plurality of feedback resistors having different values. The higher the resistance value is, the higher the gain of the amplifier. The low photocurrent levels prevent the use of solid-state relays. Therefore, gain switching requires reed relays, which would make a device employing this amplifier less rugged because any mechanical structure is easily worn out.
To reduce the wear of the relays, the transimpedance amplifier makes the measurement at the “typical” gain level. This approach increases the time spent on the measurement, which is undesirable because the specifics of the food processing industry require that tests be performed frequently and in great number.
Furthermore, the transimpedance amplifier and reed relays require separate supply voltages necessitating separate dc-to-dc converters for battery operation, which leads to increased dimensions of a testing device utilizing the transimpedance amplifier. Discrete samplings used in the testing device means momentary high light levels because of the direct exposure to room light. As a consequence, the charge is collected at a capacitor which is necessary to be discharged before the next test is conducted. Thus, a decay rate for the impedance amplifier corresponds to the fixed RC time constant making the user wait before a subsequent test can be performed.
The high front-end amplification typically makes a measuring system sensitive to environmental changes, particularly temperature drift. Typically, acquiring a baseline signal immediately before the desired signal, and subtracting the baseline signal measurement from the signal measurement correct such baseline shifts. However, this technique brings reliable results only if the baseline change is slowly varying, which is not the case with the food processing and food preparation industries where for example a cold storage room may be located next to a kitchen thus providing substantial temperature drift.
Still another measurement technique based on the photodiode detection includes a baseline measurement immediately before and after the desired signal measurement, and an average baseline signal is then subtracted from the signal measurement. Similarly to the above-discussed method, this correction can work well as long as the change in the baseline with time is nearly constant.
Finally, by monitoring the baseline signal it is sometimes possible to account for a non-uniform change in the baseline by fitting the change in the baseline to a known response function, and subtracting the calculated baseline from the measured signal.
Such procedures can only work if there is a known time period in which to acquire one or more baseline readings.
The technique of measuring the baseline before/after the sample measurement period requires the use of a shutter or some other means of ensuring that a sample is not resulting in a photodiode current, which has been associated with a few problems in case of a hand-held device. The currents generated when the shutter moved detrimentally affected the final measurement. Furthermore, both the shutter and a motor for actuating the shutter were too big to fit in the desired package. Also, the power requirements of the motor would have added substantially to the power drain placed on the batteries. Finally, the use of shutter assembly contributed to the relatively high cost and the poorer reliability.
It is known that many photodecting transducers used for the detection of luminescence are very sensitive to static charge; for instance, static charges seen when a sample consumable is inserted into the sample compartment. Conventionally, a sample compartment of known devices must be made of a conductive material or some other means must be provided to drain static charge from the sample consumable. In most cases, it is difficult to achieve the required intimate contact between the sample compartment and a conductive wall of the sample compartment to quickly drain the static charge. In other cases, it is difficult to use a conductive sample compartment, which makes it very difficult to ensure that any static charge retained on the sample consumable does not influence the signal output.
Also, a shutter is used to shield a photodiode from direct exposure to high light levels which can damage the detection circuitry and often will increase noise. Unfortunately, any static charge or potential difference on a shutter mechanism will result in a transient signal as the shutter is moved across the photodiode. Given the low-level of the photogenerated currents that must be detected, nonconductive or partially conducting surface films on the shutter can cause associated capacitances and potential differences that result in large transients that may be much larger than the photogenerated currents.
Finally, particularly in hand-held instruments of the described-above type it is difficult to fully shield the high sensitivity detection system completely to reduce spurious noise effects. This is a particular acute problem along the optical path, since most optically transparent material are nonconductors.
It is, therefore, desirable to have a hand-held device provided with a photodiode-based detection system for monitoring the ATP-luciferase reaction to provide a cost-efficient hand-held device. Furthermore, it is desirable to provide the hand-held device wherein the photodiode-based detection system for monitoring the ATP-luciferase reaction is coupled with a switched integrator to overcome the drawbacks associated with the transimpedance amplifier. Also desirable is the hand-held device wherein negative effects of static charge on a photodiode-based detection system are minimized.